As TFTs (thin-film transistors) for the active-matrix substrate of a display device such as a liquid crystal display device, amorphous silicon TFTs or polysilicon TFTs have been used, for example.
As an amorphous silicon film can be formed relatively easily, amorphous silicon TFTs can be used effectively as TFTs for a display device that needs to have a huge monitor screen. Thus, amorphous silicon TFTs are often used in the active-matrix substrate of a liquid crystal TV with a big monitor screen.
On the other hand, in a polysilicon TFT, electrons and holes have high mobility in its semiconductor layer and a large amount of OFF-state current flows through it. That is why the polysilicon TFT can charge the pixel capacitor of a liquid crystal display device and other devices in a short switching time, which is one of the advantages of the polysilicon TFT. In addition, with polysilicon TFTs, part or even all of a driver and other peripheral circuits can form an integral part of the active-matrix substrate, which is also beneficial.
A bottom-gate-type polysilicon TFT with an inverted staggered structure is disclosed in Patent Document No. 1. The semiconductor layer of that TFT has a multilayer structure in which a polysilicon film obtained by irradiating amorphous silicon with a laser beam, a hydrogenated amorphous silicon film and an n-type silicon film are stacked one upon the other. Patent Document No. 1 also discloses a semiconductor substrate on which a display area and peripheral circuits including drivers are integrated together. In that semiconductor substrate, a semiconductor layer that includes the hydrogenated silicon film but does not include the polysilicon film is used for TFTs in the display area, and a semiconductor layer that includes both the polysilicon film and the hydrogenated amorphous silicon film is used for TFTs that form the peripheral circuits such as drivers.
Patent Documents Nos. 2 and 3 disclose a semiconductor device in which a top-gate TFT with a staggered structure has been fabricated on a substrate. In that semiconductor device, TFTs in the display area (i.e., in pixel regions) have a crystallized silicon film obtained by crystallizing amorphous silicon vertically (i.e., perpendicularly to the surface of the substrate), while TFTs in the peripheral circuit area that surrounds the pixels have a crystallized silicon film obtained by crystallizing amorphous silicon laterally. As a result, according to those patent documents, a semiconductor device including TFTs with a small OFF-state current in the display area and TFTs with high mobility in the peripheral circuit area should be obtained.